The invention relates to a diffractive optical element and also an optical arrangement comprising a diffractive optical element.
A diffractive optical element is disclosed in the specialist paper xe2x80x9cZonal diffraction efficiencies and imaging of micro-Fresnel lensesxe2x80x9d in J. Mod. Opt., 45, 1405, 1998. The latter proposes reducing the structural heights of the diffraction structures with increasing radius of the Fresnel lenses therein, that is to say with decreasing structural width, and this, in the case of the optical boundary conditions chosen therein, results in an increase in the diffraction efficiency at the lens rim.
In the case of many diffractive optical elements in which, as is still being discussed, the diffraction efficiency decreases with smaller structural width, such a variation in the structural height does not, however, result in an improvement in the diffraction efficiency, with the result that the teaching of the specialist paper cannot be generalized. A diffractive optical element having constant structural heights is disclosed in U.S. Pat. No. 5,623,365 A. One of the transmissive diffractive optical elements described therein has the function of a lens having a certain focal length. This necessitates that the widths of the diffraction structures become smaller with increasing distance from the central point. The greater the desired refracting power of such a diffractive optical element with a given refractive index is to be, the greater becomes the variation in the widths of the diffractive structures with the distance from the central point and, consequently, the variation in the ratio of said widths and of the wavelength, which ratio is mainly responsible for the achievable local diffraction efficiency.
In the case of a diffractive optical element according to the type of U.S. Pat. No. 5,623,365 A, said variation in the ratio of structural width and wavelength manifests itself, as calculations based on the electromagnetic diffraction theory have shown, in that the more light is diffracted into other orders of diffraction, the narrower are the diffraction structures. This results in losses in the local diffraction efficiency in the region of narrower diffraction structures, and this results in a variation, usually undesirable, in the local diffraction efficiency of the diffractive optical element.
EP 0 312 341 A2 describes a transmissive diffractive optical element that has a plurality of concentrically disposed diffraction regions that are each designed for different illumination wavelengths and within which there is a constant structural height. Within each of said diffraction regions, therefore, the disadvantages explained in connection with U.S. Pat. No. 5,623,365 A also occur here in the case of a variation of the structural widths, and this affects the dependence of the local diffraction efficiency.
The diffractive optical element in EP 0 312 341 A2 may, in addition, have annular zones that are of opaque or partially transparent design in order to modify the light passing through so as to achieve a desired intensity distribution in the beam path downstream of the diffractive optical element. EP 0 312 341 A2 consequently discloses an optical arrangement wherein the constant structural heights of the diffraction structures for an illumination wavelength also result in the disadvantages that were discussed in connection with U.S. Pat. No. 5,623,365 A.
It is therefore a first object of the present invention to develop a diffractive optical element in such a way that its local diffraction efficiency is optimally adapted to the application purpose.
Said object is achieved, according to the invention, by a diffractive optical element having the features of the present invention.
Diffractive optical elements are used, for example, to correct for certain aberrations in an optical arrangement, for example the longitudinal color aberration, the color magnification error, the secondary spectrum and also the color variation in the coma. In addition, monochromatic aberrations may also be corrected.
The invention is based on the insight that the height of the diffraction structures can be used as a degree of freedom to modify the local diffraction efficiency of the diffractive optical element and can be altered over the area of the diffractive optical element. At the same time, it was recognized that the teaching of the specialist paper relating to reducing the structural heights so as to increase the diffraction efficiencies of less wide diffraction structures is achieved only in the case of special optical boundary conditions in which an improvement in the blaze effect is achieved by reducing the structural heights and actually results in an increase in the diffraction efficiency. In most other cases, in which the blaze effect is not improved in this way by the structural height change, the teaching of the specialist paper achieves precisely the opposite of the desired effect, namely a reduction in the diffraction efficiency at those points at which it should actually be increased according to the specialist paper, namely in the region of low diffraction structures.
The diffractive optical element according to the invention increases the comparatively low diffraction losses in the region of the wide structures and, thus, matches them to the comparatively large diffraction losses in the region of the less wide structures in such a way that a diffractive optical element results that has a local diffraction efficiency remaining constant over its area, and this is desirable for many application cases. In addition, required patterns of local diffraction efficiencies can be achieved by means of the structural height variation without substantial impairments having to be accepted in other imaging properties of the diffractive optical element in the process.
Because of the structural height variation in the region of the less wide diffraction structures, the diffractive optical element according to the invention has lower efficiency losses due to structural height than in the region of the wider diffraction structures. This is utilized to compensate completely or partly or even to overcompensate for the magnitude of the diffraction efficiency due to structural width in the region of the less wide diffraction structures that inevitably occur in the case of diffractive optical elements having constant structural height. In the case of overcompensation, the diffractive optical element has the highest diffraction efficiency at those points where, normally, the diffraction efficiency is lowest, namely in the region of the diffraction structures having the smallest widths.
In a preferred embodiment of the invention, a diffractive optical element in accordance with claim 2 has the constant pattern, particularly desired for many application cases, of the diffraction efficiency function over the area of the diffractive optical element. Said pattern is achieved by compensating exactly for the increase in the diffraction efficiency in the case of larger structural widths by a corresponding reduction in the structural height in the region of wide diffraction structures. In this connection, preferably proceeding from an optimum structural height for the diffraction structure, for which known calculating formulae exist, the structural height is reduced.
The diffractive optical element in accordance with claim 3 makes it possible to fulfill, for example, requirements relating to the diffraction efficiency function of the diffractive optical element, in which the diffraction efficiency function should increase towards the rim of the diffractive optical element. Such a diffractive optical element is able, for example, to compensate for a radially oppositely directed diffraction efficiency decrease of other optical components.
The diffractive optical element in accordance with claim 4 may be used as an apodization element.
A diffractive optical element in accordance with claim 5 can be produced with acceptable cost and is not very alignment-critical because of its rotational symmetry.
The diffraction efficiency is increased in the case of a diffractive optical element in accordance with claim 6.
The diffractive optical element can be designed as a transmissive diffractive optical element in accordance with claim 7 or as a reflective optical element, depending on application purpose.
A further object of the present invention is to develop an optical arrangement comprising a diffractive optical element according to the preamble of claim 9 in such a way that its flexibility is increased yet again if the required local total efficiencies are implemented for the optical arrangement.
This object is achieved, according to the invention, by an optical arrangement having the features of claim 9.
An additional neutral filter makes it possible to implement, for example, fine adjustments to achieve a required total efficiency function. The local total efficiency of the optical arrangement is made up in this connection of the local diffraction efficiency of the diffractive optical element and, optionally, further diffractive optical components and the local transmission of the neutral filter and, optionally, of further optical components. The transmission function of the neutral filter may, in this connection, be stepless, i.e. have a continuous transmission pattern or, alternatively, be graded, i.e. have discrete changes in the transmission.
Since neutral filters are, as a rule, less critical in the alignment of an optical arrangement, an optical arrangement can be realized in which a plurality of neutral filter shaving various transmission functions can be substituted for one another in order, in this way, to achieve different required total efficiency functions. Desired local effects that could be achieved only at higher cost by means of the variation in the structural height of the diffractive optical element can also be produced in such an optical arrangement with the aid of the neutral filter.
An optical arrangement in accordance with claim 10 is an apodization element that can be used for a number of application cases.
An optical arrangement in accordance with claim 11 has a reduced number of optical interfaces.